Japanese Patent Laid-Open No. 2007-178523 discloses a wavelength selective switch. The wavelength selective switch disclosed in Japanese Patent Laid-Open No. 2007-178523 includes a fiber array for receiving wavelength-multiplexed light; a grating for dispersing the wavelength-multiplexed light received from the fiber array; a Littrow lens for focusing the light dispersed by the grating on a predetermined position; a MEMS module having a MEMS mirror array for reflecting the light focused by the Littrow lens; and a base having these mounted thereon.